Stainless Steel Fabrication

CUTTING STAINLESS STEEL

Stainless
steels are highly alloyed materials, the various types
possess different mechanical and physical properties.
Further, these properties are, in most instances, vastly
different from low carbon (mild), medium carbon and low
alloys steels, with a corresponding effect on the cutting
methods and procedures.

It
must be emphasised that the information and recommendations
given here serve as a guide to aid in the cutting of Stainless
Steels. Many of the common problems may thus be avoided.
Procedures and results which have been successful in actual
practice should be adhered to. Experience, type and condition
of the equipment utilised may indicate slight change of
modification to the information given in this section.

SAWING
· High quality blades of High Speed Steel should
be used. Sharp teeth are essential.
· An emulsion of soluble oil is used as a cutting
fluid. More diluted emulsions are needed for cutting Austenitic
(300 series) steels to improve the cooling rate.
· All grades of Stainless Steels, both wrought
and cast, can be sawn.
· The sawing of Austenitic grades (300 Series)
is made more difficult due to their tendency to work harden.
In cutting these grades the cut must be initiated without
any riding of the saw on the work, a positive feed pressure
must be maintained, and no pressure, drag or slip should
occur on the return stroke.

HAND
HACKSAWING
Generally used for random cutting of light gauge material,
small diameter bar, tube and pipe. A blade with a wavy
set is preferable. For thin gauge sheet and thin wall
tubes a fine 32 teeth per 25mm blade is necessary. As
the thickness of the material being cut increases, the
coarseness of the blade should be increased to 24 teeth
per 25mm.

POWER
HACKSAWING
Cutting fluid should be flooded on the cut to maximise
the cooling, particularly in cutting the Austenitic grades.
More than one tooth should be in contact with the work
at all times. This necessitates small pitched blades for
cutting thinner gauges and small diameters. As the material
thickness or diameter increases the tooth spacing should
increase to give better clearance and to minimise chip
packing:

Further
the tendency of the Austenitic grades to work harden has
a significant effect on the shearing of these steels.

More
power is therefore required, and it is necessary to derate
the shears (guillotines) against their nominal capacity,
which is usually given in terms of the thickness of low
carbon (mild) steel which they are capable of shearing.

Indicative
relative derated capacities are as follows:

Low
carbon (mild) steel 10mm thick material

Corrosion Resisting Steel (3CR12) 7mm thick material

Ferritic Stainless Steel (eg 430) 7/8mm thick material

Austenitic material (eg 304) 5/6mm thick material

Corrosion
resisting (3CR12) and Ferritic Stainless Steels tend to
fracture after being cut through approximately half their
thickness. In this respect they are similar to carbon
and low alloy steels.

Austenitic
Stainless Steels are typified by a high ductility, and
hence a greater resistance to fracture. A greater degree
of penetration therefore takes place before fracture occurs.
The clearance setting of the blades is therefore important.
For shearing thin gauge sheet a clearance of 0.025 to
0.050mm is suggested.

Closer
clearance tends to induce blade wear, whereas larger clearances
allow the material being sheared to drag over to an excessive
degree, resulting in excessive wear of the blades and
a poor cut.

As
the material thickness increases the clearance should
be increased accordingly and adjusted to best suit the
specific piece of equipment being used, consistent with
minimum roll over, burr height and distortion (camber,
twist and bow).

The
nominal suggested clearances for such thicker material
are:

3CR12
Corrosion resisting steel 2.5% of material thickness

Ferritic/Austenitic
Stainless Steels 3 - 5% of material thickness

To
counteract the greater shearing force required, the hold
down pressure on the clamps may have to be increased,
particularly when shearing the Austenitic grades.

The
higher power requirements can to some extent be countered
by altering the rake/shear angle. A rake of 1 in 40 is
a shear angle of approximately 1½ °. This is
the suggested least rake which should be used. Small rake/shear
angles necessitate higher power/force, but cause less
distortion, whereas larger rakes/shear angles (eg 1 in
16 or 3½ °) reduce the power/force required,
but need higher hold down pressure on the clamps and tend
to increase distortion.

The
moving blade should be provided with as large as possible
back clearance/rake angle, without causing chipping of
this blade.

ABRASIVE
CUTTING

Abrasive
discs, rotating at high speeds, can be used for both cut-off
operations on relatively small section sizes, and for
straight line cutting of sheet and thin plate material.
* (The cutting of large radius curves may also be undertaken).

It
is therefore a useful method for cutting thinner thicknesses
to length (or to a mitre), and for making cuts of limited
length on the shop floor during fabrication.

Random
straight line cutting of sheet and this plate is normally
done dry. Vitrified or resinoid-bonded discs are used.
Care must be exercised not to induce excessive over-heating
of the cut edge.

Dedicated
discs (i.e. uncontaminated by cutting of other material)
must be used.

Random
cutting done by hand must employ safety measures, as the
discs can jam and break in the cut groove.

*
Note: Straight line cutting of thick plate (from 20 -
100mm thick) can be accomplished by abrasive cutting.
This necessitates the use of high cost, specialised equipment.

THERMAL
CUTTING

In
conventional Oxy-cutting the metal is first heated by
the flame, then an excess of oxygen is supplied. This
causes exothermic (heat generation) reactions which generate
the heat necessary to melt the oxides formed, which are
then removed from the cut by the velocity of the gas jet.

Stainless
Steels having a high level of Chromium (Cr) cannot be
cut by simple oxy-cutting methods due to the refractory
nature (very high melting point) of the Chrome Oxide which
is formed.